Operation of lean burn engines, e.g., diesel engines and lean burn gasoline engines, provide the user with excellent fuel economy, and have very low emissions of gas phase hydrocarbons and carbon monoxide due to their operation at high air/fuel ratios under fuel lean conditions. Diesel engines, in particular, also offer significant advantages over gasoline engines in terms of their fuel economy, durability, and their ability to generate high torque at low speed.
From the standpoint of emissions, however, diesel engines present problems more severe than their spark-ignition counterparts. Emission problems relate to particulate matter (PM), nitrogen oxides (NOx), unburned hydrocarbons (HC) and carbon monoxide (CO). NOx is a term used to describe various chemical species of nitrogen oxides, including nitrogen monoxide (NO) and nitrogen dioxide (NO2), among others.
The two major components of particulate matter are the volatile organic fraction (VOF) and a soot fraction (soot). The VOF condenses on the soot in layers, and is derived from the diesel fuel and oil. The VOF can exist in diesel exhaust either as a vapor or as an aerosol (fine droplets of liquid condensate) depending on the temperature of the exhaust gas. Soot is predominately composed of particles of carbon. The particulate matter from diesel exhaust is highly respirable due to its fine particle size, which poses health risks at higher exposure levels. Moreover, the VOF contains polycyclic aromatic hydrocarbons, some of which are suspected carcinogens.
Soot can be collected by wall-flow filters, which can comprise a multi-channel honeycomb structure having the ends of alternate channels on the upstream and downstream sides of the honeycomb structure plugged. This results in a checkerboard type pattern on either end. Channels plugged on the upstream or inlet end are opened on the downstream or outlet end. This permits the gas to enter the open upstream channels, flow through the porous walls and exit through the channels having open downstream ends. The gas to be treated passes into the catalytic structure through the open upstream end of a channel and is prevented from exiting by the plugged downstream end of the same channel. The gas pressure forces the gas through the porous structural walls into channels closed at the upstream end and opened at the downstream end. Such structures are primarily known to filter particles out of the exhaust gas stream. Often the structures have catalysts on the substrate, which enhance the oxidation of the particles. Typical patents disclosing such catalytic structures include U.S. Pat. Nos. 3,904,551; 4,329,162; 4,340,403; 4,364,760; 4,403,008; 4,519,820; 4,559,193; and 4,563,414.
Oxidation catalysts comprising a platinum group metal dispersed on a refractory metal oxide support are known for use in treating the exhaust of diesel engines in order to convert both hydrocarbon and carbon monoxide gaseous pollutants by catalyzing the oxidation of these pollutants to carbon dioxide and water. Such catalysts have been generally contained in units called diesel oxidation catalysts (DOC), or more simply catalytic converters, which are placed in the exhaust flow path from a diesel-powered engine to treat the exhaust before it vents to the atmosphere. Typically, the diesel oxidation catalysts are formed on ceramic or metallic substrate carriers (such as the flow-through monolith carrier, as described hereinbelow) upon which one or more catalyst coating compositions are deposited. In addition to the conversions of gaseous HC, CO and the SOF fraction of particulate matter, oxidation catalysts that contain platinum group metals (which are typically dispersed on a refractory oxide support) promote the oxidation of nitric oxide (NO) to NO2.
Catalysts used to treat the exhaust of internal combustion engines are less effective during periods of relatively low temperature operation, such as the initial cold-start period of engine operation, because the engine exhaust is not at a temperature sufficiently high for efficient catalytic conversion of noxious components in the exhaust. To this end, an adsorbent material, which may be a zeolite, may be provided as part of a catalytic treatment system in order to adsorb gaseous pollutants, usually hydrocarbons, and retain them during the initial cold-start period. As the exhaust gas temperature increases, the adsorbed hydrocarbons are driven from the adsorbent and subjected to catalytic treatment at the higher temperature.
As discussed above, oxidation catalysts comprising a platinum group metal dispersed on a refractory metal oxide support are known for use in treating exhaust gas emissions from diesel engines. Platinum (Pt) remains the most effective platinum group metal for oxidizing CO and HC in a DOC, after high temperature aging under lean conditions and in the presence of fuel sulfur. Nevertheless, Pd incorporated catalysts offer advantages in stabilizing Pt at higher temperature aging (>700° C.) and lowering catalyst cost. However, Pd based DOCs typically show higher light-off temperatures for oxidation of CO and HC, especially when used with HC storage materials, potentially causing a delay in HC and or CO light-off. Pd containing DOCs may poison the activity of Pt to convert paraffins and/or oxidize NO and may also make the catalyst more susceptible to sulfur poisoning. These characteristics have typically prevented the use of Pd as an oxidation catalyst in lean burn operations especially for light duty diesel applications where engine temperatures remain below 250° C. for most driving conditions.
It would be desirable to provide catalytic articles that addresses one or more of the disadvantages mentioned above.